I don't remember if the inside pulley half is made onto the center part or if it just slides on to the center . . . I think it's part of the center. Take it apart like Travis said . . . if the back half of the pulley won't just slide off of the center, you'll need a good three-jaw gear puller. If you don't have one and don't want to buy one, a lot of autoparts stores will "loan" you one (basically, you buy a used one at the price of a new one and they give you your money back when you return it).
Make sure the key is in it and you have it lined up when you put the new pulley on. Tap it on as far as you can with a brass, plastic, or rubber hammer. Once you get it far enough on, you can use the nut to press it the rest of the way on.
